An optical disk, such as a CD, DVD, HD DVD, or Blu-Ray disk, may contain data recorded as marks and spaces on the surface of the optical disk. The marks and spaces may correspond to zero and one bits comprising the data. After being recorded, the data may be read by detecting laser light reflected off the optical disk. The reflected light may be transformed by a photodetector to an analog replay signal. When the data is written, each mark and space may be intended to be the same nominal size on the optical disk. However, different types of optical disks and variations in the power and focus of the recording laser may cause asymmetry, where the sizes of the marks and spaces differ from their intended nominal sizes. When reading the marks and spaces, asymmetry in the replay signal may result in amplitude and duration variations and an increase in the bit error rate.
Asymmetry may be modeled and compensated for to improve the accuracy and reliability of data read from optical disks. Existing asymmetry compensation systems may use linear models to model asymmetry but may not be accurate at higher recording densities. Nonlinear models may also be used to model magnetic recording asymmetry. However, these magnetic recording models may not accurately model the nonlinearity and asymmetry of optical recording. For example, in magnetic recording, the nonlinearity is present in the magnetic read transducer that converts the magnetic data to an electrical signal. In contrast, nonlinearity in optical recording is due to variations in the marks and spaces on the surface of the disk. Some other systems may use a Volterra series nonlinear model with a maximum-likelihood sequence detector to model and detect asymmetry, but may use increased computing resources and may not be accurate for larger degrees of asymmetry. Other existing systems may compensate for asymmetry in downstream processing components, such as in a Viterbi decoder, leading to increased complexity and use of computing resources. Therefore, there is a need for a simple nonlinear model that accurately and reliably compensates for asymmetry of data read from optical disks.